Modulated carrier wave transmitter



July 30, 1940. CLOUGH ET AL 7 2,209,510

MODULATED CARRIER WAVE TRANSMITTER Filed Oct. 9, 1937 2 Sheets-Sheet 2 O Ig 4 waz/mmvs porav 77/115 7 f. GREEN.

ATTORNEY.

Patented July 30, 1940 UNITED. STATES PATENT OFFICE MODULATED CARRIER WAVE TRANSMITTER Application October 9, 1931, Serial No. 168,167 In Great Britain November 19, 1936 1 Claim.

The present invention relates to modulated carrier wave transmitters, particularly of the floating carrier type.

It is usual in floating carrier type transmitters in which the carrier power is a function of the rectified modulation envelope, to amplify the modulating potentials, which may be taken from any pair of points in the cascade speech amplifier to rectify the amplified potentials and to pass the resultant direct current after smoothing or filtering amplification, through a resistance in series with a bias source in a control circuit of the main modulation stage. The voltages across the resistance and source are opposed,

l5 and thus the total bias of the main modulator, varies with the rectified modulation envelope. Such an arrangement is shown in United States Patent Number 2,114,336, dated April 19, 1938.

Usually the modulation power is converted to 2 two-phase by means of an iron cored transformer and is rectified by means of a full wave rectifier. In order to maintain a rectilinear performance for varying power and frequency such a system has to be so operated that the maximum current passed through the iron cored transformer produces only a comparatively weak flux in the core. Thus, rectification is effected at low power and the rectified output has to beamplified.

Such a method has the disadvantage that it is expensive on account of the tube amplifiers,

and that it is difiicult to arrange the direct current amplifier so that its output can be applied to two points, both of which float with respect to ground where the modulation frequency stage is in the high potential end of the series circuit, or to a point in the high tension supply source in the modulator stage grid network.

The accompanying Figure 1 shows, in simplified diagrammatic form one arrangement of the nature referred to. The circuit of Figure 1 is, per se, known the invention being concerned with the means for applying floating carrier control potential at the terminals Ta, T4. In the system of Figure 1 the modulating stage represented by a tube V1 is in series with a modulated stage, which is, of course, a tube or bank of tubes but is represented merely by a resistance R1, the said stage V1 being connected to the positive side of its associated modulated stage R1 and floating bias for floating carrier action being applied at terminals T3, T4, across a resistance R: in series with a fixed bias, comprising a direct current source B and a speech input impedance, not shown, connected at T1, T2 between the filament 55 and grid of the modulating stage. The floating bias is generally obtained from rectified speech and applied in a sense to oppose the direct current source B, through a direct current amplifier (not shown) having its output terminals connected at T3 and Ta Neither of the points 5 T3 and T4 is at ground potential for the filament itself must float in order that the carrier amplitude may change. Therefore the whole of the direct current amplifier including its anode, filament, and bias circuits must be arranged to 10 float with respect to ground.

The present invention aims at overcoming these difilculties by eliminating the iron cored transformer and eifecting rectification at high power, preferably, though not necessarily, by means of 18 a thermionic device operating as an anode bend rectifier. Arrangements may also be made for providing different operation and release times, the usual requirement being a quick operating and a slow release time. go

In general, the fixed bias of the modulating stage is such that in the quiescent condition,

1. e., in the absence of modulation, the carrier is at a previously selected value and the anode bend rectifier is so biased that its anode curg5 rent is zero or is well down the bend.

The circuits are so arranged that when modulation potentials are applied to the rectifier, the rectifier becomes conductive on each positive half cycle and the resultant increase of rectifier curo rent increases the carrier level. Preferably a condenser is connected into the rectifier circuit and is preferably so arranged that its charge changes quickly during rectification and slowly in the continued absence of rectification, i. e., 35 of modulation.

The invention illustrated in the attached drawings wherein Figure 2 shows a high power rectifier of the anode bend type arranged in accordance with the present invention. Figure 3 illustrates an embodiment of a modulating system of floating carrier type arranged in accordance with my invention. Figure 4 is in eflect a combination of Figures 2 and 3. Figures 5, 6 and '7 are modifications of the arrangement of Figure 4, while 4 Figure 2a illustrates a form of the high frequency unit R1 of the prior modifications.

In the arrangement of rectifier circuit shown in Figure 2 the rectifier is constituted by a tube V: which has its anode connected to a suitable source B1 of positive potential through a resistance R3 and its cathode to one end of a potential source B2, modulation potentials being applied at terminals Ts, Ts. Between anode and cathode of tube V: is connected a condenser C shunted u decrease the total negative bias on tube V1.

by a resistance R2. When the rectifier tube V: is not conducting the condenser 6 is charged. when modulation potentials are applied to the grid of tube V2, the tube conducts on successive positive half cycles and the condenser charge is thus reduced at a rate which can be made rapid by suitable selection of the tube. When modulation potentials cease to be applied to the grid of the rectifier tube Va, the condenser again recharges at a rate which can be made slow. The greater the value of the resistance Ra or the value of the condenser C the slower will be the recharge rate.

The output from the rectifier V2 may be applied across a resistance connected in series with a suitable biasing potential in the grid filament input circuit of the modulating stage, i. e., R: of Figure 2 may be the same resistance as R1 of Figure 1. The modulated stage indicated diagrammatically at R1 in Figure 1 may be as indicated at R1 in Figure 2. This circuit per se has been disclosed in prior patents such as Ditcham United States Patent #2,118,l72, dated May 24, 1938, and comprises an additional tube V1 with an output circuit II) and an exciting circuit l2 the tubes output impedance being as shown in series with the output impedance of V1 and being supplied by direct current from the same source. In other words R1 of Figure 2 may replace R1 of Figures 1, 3 and 4 by substitution at points a and b of said circuits.

In the modulator system of Figure 3 where, as in Figure 1 the modulating stage V1 is in series with the modulated stage R1 there is provided an input tube V'1 the anode of which is connected to a source of positive potentials (not shown) through a resistance R1 and to the grid of the modulating stage V1, the cathode being connected to the low potential side of the modulated stage through a source of potential B1. Modulation input is applied at T1, T2 in the grid-cathode circult of tube V'1 the output from the rectifier (not shown in Figure 3) being applied at T3, T4, across a resistance R2 connected in series with the grid biasing source B of the input tube V'1. It should be noted that in this arrangement the input tube V'1 requires an increase in negative bias on modulation, to thereby render tube V'1 less conductive in order to decrease the negative bias on V1 and to increase the amount of carrier applied to the output by V1. That is the change or voltage developed across R2 when modulation occurs must be in a sense to add to the source B and increase the total negative bias on tube V'1, whereas in Figure 1 in which tube V1 in the absence of signals is biased more negatively the bias applied at T3, T1 by the rectifier should be such as to oppose the constant source B and Incidentally it may be noted that with the input tube arrangement the input tube V'1 may be arranged to have zero bias Or even positive bias in the quiescent condition.

The method of the invention may be applied in a variety of ways. Thus in the arrangement of Figure 4, which is in eifect a combination of Figures 2 and 3, V: is an anode bend rectifier tube having its anode connected through a resistance Re to the positive side of a source (not shown) of potential. Modulation potentials for rectification are applied at T5, Ts in the grid filament circuit which also includes a suitable bias source B2 and a grid resistance R5 shunted by a condenser C3. Between the anode and filament of V: are connected in parallel a resistance Ra and a condenser C1. Series connected modulating and modulated stages V1, R4 are provided, the modulating stage V1 being on the positive side of the modulated stage R1 and associated with these stages is an input tube V'1 having its anode connected to a source (not shown) of positive potential through a resistance R1. The filament 01' V'1 is connected through a source B1 of negative potential to ground, to which, also, the negative side of the modulated stage R1 is connected. This source may, in practice, be combined conveniently with the above mentioned source (not shown) which supplies the anode of the rectifier Va. The filament of the input tube V'1 is also connected to the filament or the rectifier and thus also to the other end of the resistance and condenser combination Ra, C1 associated therewith. The grid of the input tube V'1 is connected through a resistance R's and a source B of grid bias to the anode of the rectifier V1 and thus also to one end of the resistance and condenser combination R2 C1 associated with the said rectifier. Modulation potentials for modulation purposes are'applied at T1, T1, between the grid and filament of the input tube the lead to the grid including a blocking condenser G2. On the application 01' modulating potentials at T5 T6 the rectifier tube V2 conducts on positive half cycles thus shunting the resistance R1 associated therewith. The rectifier anode potential thus falls and this decrease in potential supplements the potential in B and consequently the resultant negative grid bias of the input tube V'1 increases to make the potential of the grid of V1 less negative and causes an increase of carrier levels in R1 as before. The condenser C1 smooths out changes and introduces a delay in the reduction of carrier level so that the latter does not die down quickly between periods of heavy modulation. The resistance R5 serves to bias back the rectifier tube (by virtue of a direct current drop of potential caused bygrid current) against overdriving. Thus a limit is put to the level to which the carrier can rise.

The arrangement of Figure 4 may be modified as indicated in Figure 5, which shows part of the modified circuit. Here instead of providing a separate battery B1, as the source for biasing the rectifier V2, bias is obtained from a point TP on the source B, this point being connected through a resistance RTP and resistance R5 in series to the grid of tube V2.

In a further modification partly shown in Figure 6 of the arrangement of Figure 4 the rectifier V2 has its anode connected directly to the positive side of the source of potential and its filament connected to ground and to the terminal To through a source of bias potential B2. A resistance R2 and shunt condenser 01 associated with the rectifier, instead of being connected between the anode and cathode of the rectifier are connected at one end to the cathode and at the other end are connected through a potential source B and resistance R5 to the grid of the input tube V'1 (not shown). In this arrangement modulating potentials cause the condenser C1 to charge from the source of potential connected between the anode and filament oi" of the source connected between the anode and filament of the rectifier for, in Figure 6 these two sources are effectively in series. The cathode end of the resistance R2 may then be taken to a tapping on the biasing source B2 of the rectifier, or the said end of the said resistance may be taken to a point on the source connected between anode and filament of the rectifier (e. g., by means of a tap on a potentiometer connected thereacross). In a further modification shown in Figure '7 separate bias to the rectifier is dispensed with by using a second tap on the potentiometer just mentioned. In Figure '7 the potentiometer is marked PR and the two taps PR1 and PR2, the former being connected through the resistance Re.

The filaments of the several tubes may be heated in any suitable manner, and, of course, indirectly heated cathodes may be used. The course of potential may be batteries or rectifier fed resistances with potentiometer taps or other suitable sources.

The invention, though primarily intended for the purposes hereinbefore described, is not exclusively limited to such use; for example the circuits of the invention may be employed for keying. a telegraph transmitter by audio frequency line signals, in which case the "operating and release times would be made equal.

We claim:

In a modulation system of the type wherein high frequency wave energy to be modulated flows in a stage connected in a series including the output electrodes of an electron discharge modulator tube and a source of direct current potential with means for applying modulating potentials to a control grid of the modulator tube to control its impedance and thereby control the amplitude of the high frequency currents supplied by said high frequency stage and means for additionally controlling the impedance of said modulator tube in accordance with the envelopeof the modulating potentials to thereby control the amplitude of the high frequency current comprising, a second discharge device having an anode, a cathode, and a control grid, means connecting the anode of said second tube substantially directly to the grid of said modulator tube, a connection between the cathode of said second tube and a point on said series circuit a source of potential connected with the control grid and cathode of said second discharge device said source of potential normally biasing the control grid of said device negatively relative to the cathode of said device by a potential such that it passes current and thereby maintains the control grid of said modulating tube negative and means for increasing the negative bias on said control grid of said second electron discharge device as the mean amplitude of the modulating potentials increases to thereby reduce the negative potential on the control grid of said modulator tube comprising an impedance and a condenser in parallel in the input circuit of said second discharge device, a rectifier tube having an anode, a cathode and control grid, means connecting the anode and cathode of said rectifier tube to spaced points on said impedance, and means for applying modulating potentials to the control grid and cathode of said rectifier tube.

NEWSOME HENRY CLOUGH. ERNEST GREEN. 

